1. Field of the Invention
This invention relates to a process of forming an image by silver salt diffusion transfer.
The process of forming an image by diffusion transfer using a silver salt such as a silver halide is well-known. In a general example thereof, the procedure of the process is as follows. An exposed photosensitive silver halide emulsion layer is first processed with an aqueous alkaline solution containing a developing agent and a silver halide solvent. The exposed silver halide grains are reduced to silver by the developing agent, while the unexposed silver halide grains are converted to a transferable silver complex salt by the silver halide solvent. This silver complex salt is diffused and transferred by inhibition to a layer containing silver-precipitating nuclei (an image-receiving layer) laid over the emulsion layer, where the silver complex salt is reduced by the developing agent with the aid of the silver-precipitating nuclei, to provide a silver image.
When practicing this process, use is usually made of a film unit which comprises a combination of (i) a photosensitive element containing a layer of photosensitive silver halide emulsion on a support, (ii) an image-receiving element which include an image-receiving layer containing silver-precipitating nuclei on a support, and (iii) a processing element which consists of breakable containers which hold a viscous aqueous alkaline solution containing a developing agent, a silver halide solvent, and a thickener. The emulsion layer of the photosensitive element (i) is first exposed, then the photosensitive element is placed over the image-receiving element in such a way that the emulsion layer is against the image-receiving layer of the image-receiving element (ii), and these layers are passed between a pair of rollers so that the processing element (iii) is broken and the viscous aqueous alkaline solution it contains can spread, and are then allowed to stand for a predetermined time. The image-receiving element (ii) is then peeled off the photosensitive element (i) to provide a print comprising the image-receiving layer in which the desired image has been formed.
2. Description of the Prior Art
However, during this process of diffusion transfer using a silver salt such as a silver halide, the silver image is known to exhibit a color which is not black, but is brown or some other unacceptable color. In order to eliminate this drawback, a variety of toning agents are used. 1-phenyl-5-mercapto-1,2,3,4-tetrazole is known as a typical example of such a toning agent. In addition to this, S-substituted pyrimidine derivatives as described in U.S. Pat. No. 3,756,825 are known, and their use can provide a dark-blue image of a high optical density.
However, techniques of this kind can not be said to be complete as yet, and there is room for improvement therein.
As a result of extensive studies aimed at further improving the tone of a silver image formed by the diffusion transfer process using a silver halide, the present inventors have devised this invention. Namely, we have found that the use of mercapto-triazole or a derivative thereof as a toning agent is effective for improving the tone, and have succeeded in further improving the tone of the silver image of the prior art.